This research proposal is based on the working hypothesis that carcinogenesis is a multi-staged process, involving initiation and promotion phases. We will continue to test the hypotheses that (1) initiation is due to mutation fixation, caused either by error-prone repair/replication or by other "mutator" mechanisms; and (2) promotion is the epigenetic amplification of previously mutated cells. In order to study carcinogenic initiation, we will concentrate on the basic mechanisms of mutagenesis by the use of various DNA repair deficient Chinese hamster cells, "mutator" Chinese hamster mutants, drugs and physiological conditions which could alter DNA repair, and various human genetic syndromes. In effect, we will be testing how known carcinogenic modifiers can alter mutagenesis. Furthermore, we will continue to study tumor promotion by continuing to validate our in vitro assay system for tumor promoters by testing known promoters. We will attempt to imprvoe the assay, as well as modify it for human cells. The promoter assay will also be used to study the role of cell-cell communication in the promotion process, as well as to find a human promoter. This basic knowledge will be used to develop a quantitative human transformation assay. Last, we will pursue the basic mechanism of tumor promotion via a number of collaborative projects.